Turbulence and sheared flow structures behind the isotopic dependence of the L-H power threshold on Dlll-D

摘要

Measurements of long wavelength (k⊥p_i < 1) density fluctuation characteristics in the edge of both Deuterium (D) and Hydrogen (H) plasmas across the L-H transition on DIII-D demonstrate the existence of single or double bands of low-wavenumber turbulence observed near the edge of H and D plasmas. These are strongly correlated with the L to H-mode transition power threshold (P_(LH)) and can help explain the isotopic and density dependence of Plh, and how the P_(LH) difference is reduced at higher density. Understanding and accurately predicting the L-H power threshold is critical to accessing to H-mode, and operating and achieving high confinement in burning plasmas such as ITER. Above about n_e ~ 4 x 10~(19) m~(-3), P_(LH) is seen to converge for H and D, and increases for both with higher density. Surprisingly, the P_(LH) increases significantly at low density in H but not in D plasmas. Two distinct frequency bands of density fluctuations are observed in the D plasmas at low density, n_e ~ 1.2-1.5 x 10~(19) m~3, but not in H plasmas with similar density, which appears to be correlated to the much lower power threshold in D at low density. Consistently, E x B shear in the region of rla ~ 0.95-1.0 is larger in D plasmas than in H plasmas at low density; as the P_(LH) increases with increasing density, the dual mode structure disappears while E x B shear becomes similar and small for both D and H plasmas at higher density, n_e ~ 5 x 10~(19) m~(-3), where P_(LH) is similar for both D and H plasmas. The increased edge fluctuations, increased flow shear, and the dual-band nature of edge turbulence correlating with lower P_(LH) may account for the strong isotope and density dependencies of P_(LH) and support current L-H transition theories but suggest a complex behavior that can inform a more complete model of the L-H transition threshold.